The invention relates to vehicle suspension devices. More particularly, the invention relates to vehicle suspension devices in which the main element having the xe2x80x9csuspension springxe2x80x9d function is an elastomeric flexible joint. Patent Application WO 97/47486 describes an axle of this type.
One of the advantages of using elastomeric flexible joints which act as suspension springs at the level of the joint bearings of vehicle suspension devices is to permit greater integration of the spring, shock absorber and vibration- or shock-filtering functions. According to the specifications which have to be met for the different vehicles, an axle of this type may make it possible to reduce the total number of parts used, in particular because separate filtering blocks, such as are currently used for linking the axles or the suspension arms to the body of the vehicle, are not used.
In the suspension joints, elastomeric blocks are now universally used and have supplanted the use of roller bearings or plain bearings because they are capable of imparting the required degree of freedom while ensuring good filtering which is beneficial to comfort, in particular from an acoustic point of view. Furthermore, it is possible to impart to these joints and filtering blocks relatively elaborate guiding functions by controlling their deformations under operating stresses. This is done in order to produce, for example, self-steering axles, that is to say, axles which slightly turn the plane of the wheels merely under the action of the load transfers. Another example of the possibilities provided by these elastomeric joints is to make it possible to integrate, in addition to the degree of freedom necessary for the main function of suspension, a degree of horizontal freedom which substantially improves the comfort of the vehicle.
In the case of the elastomeric joints as used to link the suspension arms (lower wishbone, trailing arms, etc.) to the body, their contribution to taking up the load is negligible (of the order of one percent). In fact, their contribution to taking up the load, which is low and inherent (since the elastomer is bound to the metal parts which surround it), does not enter into the dimensioning at all.
If it is desired to impart to such joints a real contribution to taking up the load, by therefore making them into a true suspension spring under torsional stress, a change in the trim height of the vehicle is noted over time. It is known that creep of the elastomer is one cause of this. However, it is also known that the main part of the creep takes place very quickly, that is to say, in a few days, so that it is sufficient to make allowance for this when mounting the spring so that the vehicle trim height after a few days is the height intended when the vehicle was designed. This, in practice, consists of selecting the mounting azimuth while taking into account both the rated load of the vehicle and the creep.
Unfortunately, there are still variations in vehicle trim height which cannot be explained by these phenomena. These variations appear to be erratic. Shifts in vehicle trim height which may be of as much as several centimeters have been noted. This problem is a great nuisance in particular for final adjustment operations for the geometric parameters which depend on the vehicle trim height or trim (that is to say, the angular position of the body relative to the horizontal ground) of the vehicle. This may be the case, for example, for the orientation of the beam of the headlights or of the brake power distributor.
It has been discovered that the hysteresis of the elastomer is one of the causes of these variations. The hysteresis, which is a phenomenon which is well-known per se, means that the apparent stiffness of the material is different during the increase in the stress and during the reduction in this stress. The characteristic xe2x80x9coutwardxe2x80x9d and xe2x80x9creturnxe2x80x9d stress/strain curves are not superposed as, for example, for a conventional metal spring. It has been discovered that, as long as the elastomeric spring has not been stressed as far as the bump stop, the vehicle trim height (under equal load) is not stabilized. It would appear that the extent of this phenomenon is due to the fact that the starting part of the operation of the spring, that is to say, the state which is free of any stress, is very far from the zone of the stress/strain graph which is covered by the elastomeric spring during operation on a vehicle. Said zone of the graph is displaced substantially each time the spring reaches a xe2x80x9cnewxe2x80x9d maximum deformation, greater than the maximum deformations previously achieved. In fact, it is noted that the mechanical characteristics of the elastomeric spring become substantially constant over time only once the spring has reached (at least once) its state of maximum deformation. It is this observation which forms the basis of the principle of the invention, according to which the joint bearing a load, in order to stabilize its point of operation, must undergo deformation comparable to the maximum deformation provided for during operation.
The invention therefore relates to a process for stabilizing the mechanical operating characteristics of an elastomeric flexible joint, said joint being intended to act as a spring in a suspension device for a vehicle, said vehicle having a body and wheels, said flexible joint being intended to take up a given rated load, said suspension permitting movement as far as a maximum compression limit, said process consisting, before the putting into normal service of the vehicle, of subjecting said joint to a deformation R comparable to a maximum deformation M corresponding to the deformation undergone by said joint when said suspension reaches said maximum compression limit.
Preferably, this process is characterized in that said deformation R is greater than or equal to said maximum deformation M.
Preferably, this process is furthermore characterized in that said flexible joint is subjected to said deformation R, then said deformation R is relaxed until a deformation S is achieved which corresponds substantially to said rated load.
Preferably, the process of the invention is also characterized in that, said vehicle having a vehicle trim height relative to the ground, said flexible joint is subjected to said deformation R during the final assembly of the vehicle and before adjustments linked to said vehicle trim height are carried out.
Preferably, the process of the invention is characterized in that said deformation R is achieved, after mounting said suspension device on the vehicle, by making said suspension device undergo a movement which extends as far as said maximum compression limit.
Preferably, this process is furthermore characterized in that, the vehicle being on the ground, said deformation R is obtained, after mounting said suspension device on the vehicle, by exerting an appropriate force on said body of said vehicle.
Alternatively, the process of the invention is characterized in that said deformation R is obtained, after mounting said device on the vehicle, by exerting a force on the wheels of said vehicle, the body of which is kept immobilized.
Alternatively, the process of the invention is characterized in that said flexible joint is subjected to said deformation R before the mounting of said suspension device on said vehicle.
Preferably, this process is furthermore characterized in that said flexible joint is subjected to said deformation R before said flexible joint is mounted within said suspension device.
The invention also consists of an elastomeric flexible joint intended to act as a spring for a vehicle suspension device, said joint bearing a substantial part of the load, said flexible joint being characterized in that it comprises means suitable for keeping it in a temporary state of preselected deformation, said preselected deformation being of the same type as a deformation caused by said load.
Preferably, the flexible joint of the invention is characterized in that it comprises cutouts in the elastomeric material and that wedging means are present in said cutouts.
Alternatively, the flexible joint of the invention is characterized in that, said flexible joint comprising rigid parts which are mobile relative to each other, said flexible joint is kept in said temporary state of preselected deformation by a clamping means temporarily linking said rigid parts.
Thus, the invention proposes subjecting the elastomeric spring, before the mounting thereof, during the mounting thereof and in any case before being put into normal service, that is to say, delivery of the vehicle to the end user, to comparable deformation, that is to say, deformation of the same type, of the same direction and of an intensity similar to the maximum deformation conceivable in service. In practice, the definition of the maximum deformation conceivable in service will depend on numerous parameters because, even in the case of a suspension device comprising mechanical displacement stops, the displacement limit cannot be located precisely because it is a function of the deformations (elastic or plastic) of the stops and of the rest of the structure. However, it is possible to agree on a maximum deformation corresponding, for example, to the deformation beyond which the rigid structure of the suspension device deforms plastically. It is also possible to agree that the maximum deformation is achieved when the wheel comes into contact with the wheel housing formed in the vehicle body if this eventuality is reasonable. This precise choice of the maximum reference deformation is arbitrary, but the deviations are limited in terms of deformation of the spring (for example, at most of the order of 10%). The principle of the invention is to subject the joint to a deformation such that its point of equilibrium around the static load is stabilized. This aim is achieved by the fact that the joint is subjected to deformation comparable to the maximum deformation envisaged in normal service. Preferably, in order to limit further the instability of the static point of equilibrium of the suspension, care will be taken to subject the joint to an equal or slightly greater deformation (for example of the order of the uncertainty on the maximum conceivable deformation).
Tests have shown that deformation for a very short time (of the order of one second) was sufficient to obtain the desired effect. It was also noted that the fact of maintaining this deformation for a longer time (several days, or even several weeks) did not have any adverse consequences.
The invention proposes various embodiments of this solution, the common principle being to subject the flexible joint to a deformation R comparable to the maximum deformation M envisaged in service on the suspension device and, preferably, then to relax said stress, in order to achieve a state of deformation S corresponding substantially to the rated load intended for the vehicle. The suspension devices (or the vehicles intended to be fitted therewith) generally comprise a stop limiting the movement of the suspension, that is to say that the conceivable maximum deformation M is substantially (see above) determined by the geometric characteristics of design of the vehicle and that it is made concrete on the vehicle.
A first possible embodiment of the invention consists, after mounting the axle on the vehicle, in stressing the suspension so as to bring it to the displacement limit, at the maximum compression permitted by the geometry (and, if applicable, the reasonable deformations of the structure or stops) and then, preferably, let the vehicle rest on its wheels, the elastomeric spring bearing the rated static load.
A second possible embodiment of the invention consists, after assembling the constituent parts of the suspension device but before mounting it on the vehicle, in appropriately stressing the axle to subject it to the deformation R, relaxing this stress to the level of deformation S (substantially corresponding to the take up of the rated static load), then locking the suspension arms relative to a chassis reference of the vehicle until this subassembly is mounted on the vehicle and preferably until it is placed under static load.
A third possible embodiment of the invention consists in imposing the deformation R on an elastomeric flexible joint before it is mounted within a suspension device, then keeping it deformed at the level of deformation S by a device for locking the joint until it is mounted and preferably until it is placed under rated load.
In fact, the process of the invention may be implemented at any stage of assembly of the vehicle before it is put into service. However, the numerous conceivable variants do not have the same advantages and the same constraints.
In the case of suspension joints having cutouts in the elastomeric material, as described in patent application EP 0 956 984, the cut-outs in the elastomeric material deforming under the stress, wedging means may be used as the means for locking the joint, the profiles of which wedging means correspond to those of the deformed cutouts. These wedging means are then placed in the cutouts before completely releasing the stress, then these wedging means are removed after mounting on the vehicle, preferably after it is placed under static load.
In the case of a suspension device also comprising a shock-absorber, it is possible to maintain the stress of the elastomeric spring by temporarily locking the movement of the suspension by means of mechanical or hydraulic limitation of the displacement of the shock-absorber.
It is also possible to adopt any other mechanical locking solution which angularly clamps the radially outer and inner rigid parts of the elastomeric spring relative to each other, then the parts are freed after the spring has been mounted on the suspension device, preferably on the vehicle after it is placed under static load.
Generally, it is preferred to lock the flexible joint at a level of deformation S which is substantially equal to the deformation corresponding to the rated load. But for various reasons, a substantially different level of deformation may be selected, for example a lower level of deformation (lower stress, hence potentially lower cost of the holding elements and less energy stored) or a greater level of deformation (better anticipation of later creep, less space taken up).
The solutions intended to stabilize the mechanical operating characteristics of a joint well before mounting (several days) on the vehicle have additional advantages: firstly, the creep of the first few days is integrated (see above), which makes it possible to effect final adjustment of the vehicle (adjustment of the headlight beams, of the brake power distributor, the front axle, etc.) in a configuration even closer to the later operating situation and secondly, this permits mounting (during a repair) of such a joint by a normally-equipped professional without occurrence of the phenomenon of variation in vehicle trim height described above.